In Hong Kong, it is very easy to get pre-paid SIM card locally so you don't need to get it ahead of the trip. The mobile carriers are Orange, Sunday, People. You can find the pre-paid SIM cards in convenient stores or book stores at the airport, or numerous small cell phone shops through out Hong Kong. The cost is HK$100 (about US$12.5) and it gives you approx 300 min of local calls. The SIM cards are also available in higher denominations for more covered minutes. I am not sure about the international call rates but it is on the small booklet that comes with the SIM. If it is expansive, you can always get the phone cards that give you very low international rate to use on any land line or cell phones. Most pre-paid SIM you get in Hong Kong only work in HK, but there are a few special kinds that work both in HK and China (of course less minutes). Just ask the sales person about it.